Apparatus for forming liquid droplets having a mechanically fixed inner microtube

ABSTRACT

The invention relates to an apparatus for forming liquid droplets, such as a micro nebulizer, useful for preparing samples for subsequent analysis via MS, AA, ICP, CE/MS, and similar analytical systems. The apparatus has a mechanically stabilized inner microtube or needle, thereby ensuring controllably uniform droplet size. The mechanical stabilization is provided by securing the inner microtube or needle, such as by narrowing the inner diameter of the outer microtube or otherwise narrowing the annular intermediate space between the inner and outer microtubes for a predetermined length. Thus, the inner microtube is secured in a centered or otherwise predetermined fixed radial position, with minimum perturbation of the fluid flow. Further, a tip, when coupled with the exit end of the outer microtube, provides a region in which the sheath fluid flow in the outer microtube stabilizes prior to both exiting the tip and colliding with the liquid analyte exiting the inner microtube.

This is a continuation of application Ser. No. 08/722,644 filed on Sep.27, 1996, U.S. Pat. No. 5,868,322, which was a continuation-in-part ofapplication Ser. No. 08/593,319 filed on Jan. 31, 1996, abandoned.

FIELD OF INVENTION

This invention relates generally to apparatus for forming liquiddroplets, such as liquid sprayers, atomizers, and the like. Moreparticularly, this invention relates to nebulizers useful in liquidchromatography (LC) or capillary electrophoresis (CE) coupled with ananalytical system, such as a mass spectrometer (MS).

BACKGROUND

Micro nebulizers have been used to convert liquid samples to finedroplets suitable for analysis. Micro nebulizers provide a usefulinterface for analytical systems based on techniques such as massspectrometry (MS), atomic absorption (AA), or inductively coupled plasma(ICP) which cannot directly analyze liquid samples. In such analyticalsystems, the liquid sample must first be converted to a gas. The idealconversion would, theoretically, involve spraying the liquid intouniform fine droplets. The uniform fine droplets then would then bedried and converted to a gas suitable for analysis. In practice, uniformfine droplets are difficult to attain. If droplets vary in size, theheat necessary to dry a larger droplet damages the analyte in a finerdroplet. Large droplets, if left undried, result in noise and signalinterference.

Current nebulizers rely on a concentric microtube arrangement to sprayliquid samples into droplets. The inner microtube carries the liquidsample; the outer microtube carries an inert fluid (liquid or gas) usedas a sheath fluid. At the exits of the concentric microtubes, the liquidsample and the sheath fluid collide and the liquid sample is broken intodroplets by the shearing force of the sheath fluid. Uniform laminarsheath fluid flow is critical to producing uniform size droplets. Anyimperfections in the annular region between the inner and outermicrotubes forming the sheath fluid flow region create turbulence in thesheath fluid, which translates directly into lack of control of dropletsize and uniformity. Such imperfections may be generated, for example,by transition points within the sheath fluid flow region such as at thepoint the sheath fluid is introduced into the outer microtube.

To compensate for such imperfections in current nebulizer microtubes,nebulizers with microtubes of relatively great length have been used.The increase in microtube length (in some cases up to 25 mm or more)permits the sheath fluid to stabilize after the turbulence induced byinternal imperfections in the sheath fluid entry point transition.However, increased microtube length alone fails to solve the problementirely or even satisfactorily. Long microtubes dissipate the energyneeded for the shearing force collision of sheath fluid and the liquidsample. More problematic is that long concentric microtubes do not staycentered relative to each other; thus, the exit aperture experienced bythe sheath fluid is either asymmetrical, changes with time, or both. Asa result, the velocity and shearing force of the sheath fluidexperienced by the liquid sample is unevenly distributed and changeswith time, which brings about the problem that plagues currentnebulizers: namely, variation in size and uniformity of the dropletsproduced.

What is needed is a nebulizer that reproducibly generates uniform finedroplets of controllable size and distribution. Further, what is neededis a nebulizer wherein the aperture experienced by the sheath fluid iscontrollable. Also desirable is a nebulizer wherein the inner microtubeor needle is mechanically stabilized and wherein such stabilizerelements do not substantially impede the sheath fluid flow in the outermicrotube. Further, what is needed is a nebulizer wherein the sheathfluid flow path is sufficiently short and smooth such that the reductionof energy associated with liquid droplet formation occurs substantiallyat or near the point of nebulization.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides an apparatus for formingdroplets from a liquid comprising:

A. at least one inner microtube having an outer wall, an exit end, andan exit end aperture,

B. an outer microtube having an inner wall, an exit end, and an exit endaperture, wherein the inner microtube has an outer diameter smaller thanthe inner diameter of the outer microtube, and the inner microtube ispositioned within and is surrounded by the outer microtube such that anannular intermediate space is formed therebetween, with the exit end ofeach microtube being located at a same end of the apparatus,

C. one or more intermediate structures either (i) extending inwardradially from the inner wall of the outer microtube and contacting theouter wall of the inner microtube for a predetermined length, (ii)extending outward radially from the outer wall of the inner microtubeand contacting the inner wall of the outer microtube for a predeterminedlength, or (iii) spanning the annular intermediate space and contactingboth the outer wall of the inner microtube and the inner wall of theouter microtube for a predetermined length, wherein the intermediatestructure is situated so as to mechanically stabilize the innermicrotube, and

D. one or more communicating channels continuing lengthwise along theoutside of the inner microtube, wherein the communicating channelprovides a continuation of the annular intermediate space and throughwhich a fluid may continue to flow after encountering the intermediatestructure.

In a preferred embodiment, the invention provides a nebulizer assembly,comprising:

A. at least one inner microtube having an exit end and an exit endaperture,

B. an outer microtube having an exit end adapted for coupling with a tipand an exit end aperture, wherein the inner microtube has an outerdiameter smaller than the inner diameter of the outer microtube, and theinner microtube is positioned within and is surrounded by the outermicrotube such that an annular intermediate space is formedtherebetween, with the exit end of each microtube being located at asame end of the nebulizer assembly, and

C. a tip which couples with a surface at the exit end of the outermicrotube, thereby forming a region near the exit ends of the microtubeswithin which fluid flow may stabilize; wherein the inner diameter of theouter microtube is narrowed for a predetermined portion of its lengthsuch that the annular intermediate space is reduced to one or more fluidcommunicating channels.

In one embodiment, two or more inner microtubes are included within theapparatus or nebulizer assembly, such that multiple annular intermediatespaces for fluid flow are formed.

The invention provides an apparatus such as a nebulizer thatreproducibly generates uniform droplets of controllable size anddistribution. Further, the invention provides an apparatus wherein theaperture experienced by the sheath fluid is controllable. The inventionfurther provides an apparatus wherein the inner microtube or needle ismechanically stabilized and wherein such stabilizer elements do notsubstantially impede sheath fluid flow in the outer microtube. Further,the apparatus taught herein provides a sheath fluid flow path which issufficiently short and smooth such that the reduction of energyassociated with liquid droplet formation occurs substantially at or nearthe point of nebulization.

The apparatus described above may be coupled to any analytical system inorder that droplets produced by the apparatus may be analyzed. In apreferred embodiment, the apparatus is coupled to a mass spectrometer.The invention is particularly useful as part of an interface for massspectrometers employing, for example, electrospray ionization (ESI) oratmospheric pressure chemical ionization (APCI) to analyze liquidsamples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, A through C, schematically illustrates a nebulizer assemblyaccording to the present invention.

FIG. 1A is an enlarged schematic of the inner stabilizer portion of anebulizer;

FIG. 1B is a cross section of the nebulizer of FIG. 1A taken along axisAA; and

FIG. 1C is an enlarged view of the tip of the nebulizer assembly of FIG.1A.

FIG. 2 illustrates the tip and outer microtube components of anembodiment of the invention taught herein.

FIG. 3, A through D, illustrates embodiments of the invention employingtwo inner microtubes and an outer microtube.

FIG. 4 illustrates the angle, a, formed by the intersection of thecenter axis of the inner microtube and a line drawn tangent to thechamfer on the outer wall of the inner microtube.

FIG. 5, A and B respectively, illustrates an alternate embodiment of theinvention (controlled asymmetrical flow) and an end section thereof.

FIG. 6 schematically illustrates an alternate embodiment of the presentinvention (CE/MS extended needle with liquid sheath flow).

FIG. 7 illustrates representative results obtainable by the nebulizer ofFIG. 1 under conventional APCI conditions.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides an improved apparatus for reproducibly formingand controlling size and uniformity of droplets from a liquid. Theinvention provides radial and angular control of sheath fluid flow pathsand exit apertures, thereby producing controlled sheath fluid flowvelocity and distribution and resultant droplet size and uniformity. Theapparatus has a mechanically stabilized inner microtube or needle, whichis provided by securing in a predetermined fixed position the innermicrotube, such as by narrowing the inner diameter of the outermicrotube or otherwise narrowing the annular intermediate space betweenthe inner and outer microtubes for a predetermined length. Thus, theinner microtube is secured in a centered or otherwise predeterminedfixed position, with minimum perturbation of the sheath fluid flow.Further, an optional tip, when coupled with a surface at the exit end ofthe outer microtube, provides a region in which the sheath fluid flowfrom the outer microtube stabilizes prior to both exiting the tip andcolliding with the liquid sample (analyte) exiting the inner microtube.

Broadly, the invention provides an apparatus comprised of at least oneinner microtube and an outer microtube, the inner microtube positionedwithin and surrounded by the outer microtube and defining an inter tubeannular intermediate space therebetween. Preferably the inner and outermicrotubes are positioned in a concentric arrangement about a commoncentral axis. However, it is operable and in certain embodiments it maybe desirable to "offset" the position of the inner and outer microtubesin an eccentric arrangement about substantially parallel axes, such asillustrated in FIG. 5. Each microtube has an exit end and an exit endaperture, through which fluid flows out of each of the microtubes. Theliquid sample or analyte flows through the inside of the innermicrotube, while the sheath fluid flows inside of the outer microtubeand outside of the inner microtube within the annular intermediatespace. When more than one inner microtube is used, multiple annularintermediate spaces are formed, wherein more than one sheath fluid or amake-up fluid may be employed.

The outer microtube is adapted so that for at least a portion of itslength, the annular intermediate space is reduced to one or more fluidcommunicating channels and the remainder of the annular intermediatespace comprises one or more intermediate structures. The inner microtubeis secured in a substantially fixed radial position by providing anintermediate stabilizing brace-like, extension-type, or bridgingstructure in the annular intermediate space formed between the innermicrotube and the outer microtube. The intermediate structure isgenerally fluid impermeable and extends for a predetermined portion ofthe length of the outer microtube and mechanically provides support forthe inner microtube, stabilizing the inner microtube into a fixed radialposition and optionally a fixed axial position. In one embodiment, thebrace-like intermediate structure extends inward and contacts the outerwall of the inner microtube. Alternatively, the intermediate structuremay be provided by an extension radially outward from the outer wall ofthe inner microtube contacting or sealably abutting the inner wall ofthe outer microtube. In still another embodiment, the intermediatestructure bridges or spans the annular intermediate space and contactsboth the outer wall of the inner microtube and the inner wall of theouter microtube. The annular intermediate space also contains one ormore preferably substantially parallel fluid communicating channelscontinuing lengthwise for a portion of the outer microtube, which permitsheath fluid flow therethrough. In any of the embodiments, theintermediate structure serves the function of stabilizing the innermicrotube by restricting radial movement of the inner microtube, whileat the same time permitting sheath fluid flow by virtue of a preferablysubstantially parallel channel or channels communicating with theannular intermediate space on either side of the stabilizingintermediate structure. When more than one inner microtube is employed,generally the outermost inner microtube, that is, the inner microtubeclosest to the outer microtube, is mechanically stabilized.

As depicted in FIG. 1, a preferred nebulizer assembly in accordance withthe present invention comprises an inner microtube 12 having a firstexit end aperture and an outer microtube 14 surrounding the innermicrotube 12 and defining an inter tube annular intermediate space 24Atherebetween. The outer microtube 14 has a second exit end aperture atthe same end of the nebulizer assembly and preferably at about the sameplace as the first exit end aperture of the inner microtube 12. Theouter microtube 14 has, for a lengthwise portion 16 of the outermicrotube preferably extending up to about 1.1 mm from the second exitend aperture, a reduction of the annular intermediate space to one ormore fluid communicating channels (FIG. 1B, 18). The reduction of theannular intermediate space is provided by means of one or moresubstantially impermeable intermediate structures 20 which extend fromthe inner wall of the outer microtube 14. The intermediate structure orextension 20 contacts the outer wall of the inner microtube 12 for apredetermined length and functions to mechanically fix in radialposition the inner microtube 12. The channel 18 provides the only meansfor passage of sheath fluid continuing lengthwise along the outside ofthe inner microtube and provides a continuation of the annularintermediate space between the outer microtube 14 and inner microtube 12through which sheath fluid may continue to flow after encountering theintermediate structure or extension 20 which effectively narrows theannular intermediate space.

In one preferred embodiment, the invention further provides a tip which,when coupled with a surface at the exit end of the outer microtube,provides a region into which the sheath fluid flow from the outermicrotube (restricted to the communicating channels between theintermediate structure) may expand and establish stable fluid flowdynamics (wherein both gas and liquid are termed a fluid for purposes ofthis invention). Use of such a tip is desirable for the followingreasons. Sheath fluid flow within the annular intermediate space priorto encountering the intermediate structure is preferably laminar.Turbulence may occur as the sheath fluid passes through thecommunicating channel(s). Nebulizing shearing forces would be adverselyaffected if the sheath fluid flow were not laminar. Therefore, the tip,when coupled to a surface at the exit end of the outer microtube,provides a region into which the sheath fluid may flow and reestablishlaminar flow prior to nebulization. The stable (laminar) sheath fluidflow contributes to efficient droplet formation by permitting the energyof the sheath fluid molecules, at the point of nebulization, to pass tothe liquid and result in droplet formation, rather than being dissipatedin friction inside the microtubes. The tip, upon coupling with a surfaceat the exit end of the outer microtube, encircles the inner microtubeand preferably provides for a length of about 1.0 mm of an annularintermediate space similar to that of the first portion of the outermicrotube, providing thereby a buffer space within which the sheathfluid flow can restabilize prior to exiting through the tip orifice.

As illustrated in FIG. 1, an insertable cap or tip 22 couples pressablywith a surface at the exit end of the outer microtubes 14, encirclingthe inner microtube 12 and providing an annular intermediate space 24Bbetween the inner wall of the tip 22 and the outer wall of the innermicrotube 12 for a length of about 1.0 mm. The tip 22 providesconditions amenable to stabilized fluid (gas or liquid) dynamics, aneffective "buffer" region, within which fluid flow may stabilize oroptimize prior to exiting through the tip end orifice 26. FIG. 2illustrates the receiving end 28 of the outer microtube 14 adaptablyconfigured in order to receive the insertable tip 22. Alternately, thetip may be configured as a cap or extendable sleeve (not shown), whichmounts onto or over and around the outer microtube 14. However, theinsertable tip illustrated in FIG. 2 is preferred for productionpurposes.

The exit end of the inner microtube may protrude beyond (as illustratedin FIG. 6), be flush with, or be recessed with respect to (asillustrated in FIG. 1) the exit end of the outer microtube. For highliquid sample flowrates, such as typically experienced in APCI-MSapplications, the exit end of the inner microtube is preferably recessedwith respect to the exit end of the outer microtube. For low liquidsample flowrates, such as typically experienced in CE-MS and some ESI-MSapplications, the exit end of the inner microtube preferably is flushwith or protrudes beyond the exit end of the outer microtube. In certainembodiments, all, some, or one of the outside surfaces of the tip, theexit ends of the inner microtube(s), and the exit end of the outermicrotube, are preferably chamfered, angled, or otherwise tapered.Tapering the inner microtubes may be particularly important with lowliquid sample flowrates such as experienced in CE-MS. For example, inCE-MS applications, tapering the inner microtubes assists in focussingthe generated electrical field, thus improving sensitivity and stabilityat low liquid sample flowrates. In certain other embodiments, theposition of the inner microtube, while fixed in the radial directionrelative to the outer microtube, is adjustable along its longitudinalaxis by providing an adjustment means such as a threaded coupling to theinner microtube.

In certain embodiments, more than one inner microtube may be used. Inthe embodiment illustrated in FIG. 3 (A through D) the nebulizercomprises two inner microtubes. The first inner microtube 30 ispositioned within and surrounded by the second inner microtube 32, thusdefining a first inter tube annular intermediate space 34 therebetween.The second inner microtube 32 is positioned within and surrounded by theouter microtube 36, thus defining a second inter tube annularintermediate space 38A and 38B therebetween and having four (4) radialcommunicating channels 44. Each microtube has an exit end and an exitend aperture, through which fluid flows out of each of the microtubes.The liquid sample or analyte flows through the inside of the first innermicrotube 30, while a sheath fluid or make-up fluid flows inside of thesecond inner microtube 32 and outside the first inner microtube 30within the first annular intermediate space 34. A sheath fluid flowsinside of the outer microtube 36 and outside the second inner microtube32 within the second annular intermediate space 38A and 38B. Theembodiment illustrated in FIG. 3 also comprises intermediate structures40 and a tip 42, as discussed herein above.

FIG. 3B depicts an enlarged view of the exit end of the nebulizer. Thefirst and second inner microtubes 30 and 32 are tapered to increase theelectrical field gradient at the tip 42 in order to assist in Taylorcone formation of the electrosprayed liquid sample. The first annularintermediate space 34 is shown, as well as the details of tapering atthe exit ends of the first and second inner microtubes 30 and 32. FIG.3C depicts an enlarged view of an alternative embodiment using differenttapering geometries for the first and second inner microtubes 30 and 32for easier fabrication, relaxed assembly tolerances, and improvedstability. The first inner microtube 30 protrudes from the exit end ofthe second inner microtube 32 to help initiate the Taylor cone and toavoid signal instability. In a preferred embodiment, the first innermicrotube is fabricated from glass and the second inner microtube isfabricated from metal such as stainless steel and serves as theterminating CE electrode. In CE with low liquid sample flowrates, if thefirst inner microtube were recessed into the second inner microtube, theCE current would cause bubble formation, resulting in undesirable signalinstability. Conversely, the maximum acceptable protrusion of the firstinner microtube past the second inner microtube is limited by theconfines of the sides of the fully formed Taylor cone at the desiredliquid sample flowrate.

The embodiment illustrated in FIG. 3 is particularly useful, forexample, in CE-MS, wherein make-up fluid, typically a liquid, isemployed in the first annular intermediate space 34 and a sheath fluid,typically a gas, is employed in the second annular intermediate space38A and 38B. As previously disclosed, all, some, or one of the outsidesurfaces of the tip, the exit ends of the inner microtubes, and the exitend of the outer microtube are advantageously tapered.

Microtubing generally suitable for any currently practiced micronebulizer may be adapted for the invention herein. For example, in apreferred embodiment, the outer microtube comprises an originally solidstainless steel rod drilled out to an inner diameter of about 1.6 mm onone end to a depth of about 12 mm. The other end of the rod is drilledout to approximately the same inner diameter for a depth of about 1 mm.For the remainder of the intermediate solid rod portion (about 1.6 mm),a center hole is drilled of sufficient diameter to accommodate the innermicrotube; multiple channels extending radially, for example three (3)or four (4), are drilled or otherwise machined, optionally eachequidistant from the other. Such a geometry may be selected to promotebalanced sheath fluid flow, as well as to provide for easy and certaininsertion of the needle-like inner microtube into the outer microtube ofthe nebulizer. The radial openings (see, for example, FIG. 1B) aresufficiently narrow in opening width so as not to permit the innermicrotube or needle to pass anywhere but directly into the center hole.Although any suitably refined micro drilling technique may suffice, finewire electrical discharge machining (EDM) is preferred. Alternately, aplunge quill technique may be used but is slower and more costly.

The invention is not limited to an intermediate stabilizing structureprovided by means of drilling out the outer microtube. It is alsopossible to adapt the inner needle or microtube so as to provideprojections from the outside wall of the inner needle or microtube.These projections provide the intermediate structure and the mechanicalstabilization and are considered alternate embodiments of the inventiontaught herein. Microtubes with an intermediate stabilizing structure inthe inter tube annular intermediate space by whatever means constructedare considered to be within the scope of the invention claimed herein.

The inner microtube in a preferred embodiment comprises needle gaugestainless steel or fused silica cut to a desired length and chamfered,angled, or otherwise tapered on the outside surface of the tip of thefree end. In one preferred embodiment, the inner microtube is, forexample, a 33 gauge needle or other microtube device with an innerdiameter of about 0.004 inches (0.1 mm). The angling, chamfering, ortapering such as illustrated in FIG. 4 may be accomplished by chemicaletching. While any angle or radius less than or about ninety (90)degrees is helpful in directing sheath fluid flow at the exit endaperture, an angle of about thirty (30) degrees performs well in apreferred embodiment. The angle, α, is measured by the angle formed bythe intersection of the center axis of the inner microtube and a linedrawn tangent to the chamfer, angle, or taper on the outer wall of theinner microtube.

The insertable tip (see, for example, FIG. 1C) is also preferably ofstainless steel, drilled to be of outer diameter sufficient to press fitinto the exit end opening of the outer microtube, for example, an innerdiameter of about 0.80 mm to about 0.89 mm, preferably of about 0.84 mm,and an exit orifice. The tip may be hand inserted or a pin vise and Vblock used; a die and arbor press are preferred for production assembly.Upon insertion, contact is fluid tight and no gas or liquid may passbetween the tip and microtube surfaces so contacted.

All dimensions used herein are suggestive and not intended to berestrictive. Appropriate aperture sizes may be any that generallycorrespond to flow rates useful for nebulization. The relative lengths(microtube, intermediate structure, tip) have been empiricallydetermined. In general, the length of the nebulizer should be as shortas is effective, with sufficient tip length to stabilize inert sheathfluid flow after exiting the communicating channel(s) through theintermediate structure. The length of the intermediate structure orlength for which the inner diameter of the outer microtube is narrowedis preferably is about four (4) to ten (10) times the diameter of theinner microtube, in order to provide adequate stabilizing of the innermicrotube under conditions of operation.

During operation, the liquid sample or analyte flows through the insideof the inner microtube, while the sheath fluid flows inside of the outermicrotube and outside the inner microtube within the annularintermediate space. When more than one inner microtube is used, multipleannular intermediate spaces are formed, wherein more than one sheathfluid or a make-up fluid may be employed. Typical flowrates depend uponthe application. For example, in ESI-MS and APCI-MS, typical liquidsample flow rates within the inner microtube are in the range of fromabout 1 microliter/minute to about 2,000 microliters/minute inclusive;sheath fluid flow rates in such applications are typically in the rangeof from about 2 liters/minute to about 6 liters/minute inclusive. InCE-MS, typical liquid sample flow rates within the inner microtube areless than or equal to about 1 microliter/minute such as about 500nanoliters/minute to about 1 microliter/minute inclusive. Frequently,however, CE-MS applications will employ nebulizers having at least twoinner microtubes providing at least two annular intermediate spaces, thefirst annular intermediate space providing for make-up fluid flow(typically a liquid) and the second annular intermediate space providingfor sheath fluid flow (typically a gas). In such applications, thecombined liquid sample and make-up fluid flow rate will typically beless than or equal to about 1 microliter/minute, with sheath fluid flowrates in such applications typically in the range of from about 2liters/minute to about 6 liters/minute inclusive.

FIG. 5 illustrates an alternate embodiment of the invention taughtherein referred to as "controlled asymmetry". In some applications, onlya portion of the eluent will be analyzed rather than the entire sample.In such applications, only a controlled portion of the droplets producedneed be fine, but it is desirable to controllably create reproduciblyfine droplets for analysis. FIG. 5B illustrates an end-view ofasymmetrically drilled exit aperture 28 and internal stabilizingelements (not shown). In operation, the gas velocity is greatest in thewidest gap portion of the aperture 29, and there the sheath gas willshear the liquid so as to generate the finest droplets. Droplets locatedon that side of the exiting plume will be selected for analysis, and allothers droplets will pass by.

FIG. 6 illustrates an alternate embodiment of the invention taughtherein, namely, a configuration useful in CE/MS. In CE/MS, the outermicrotube conducts, not a sheath gas as in most other nebulizerapplications, but a sheath liquid. The inner microtube is fabricatedfrom a nonconducting material. The sheath liquid in the outer microtube14 encounters the analyte liquid at the exit end aperture, where thesheath liquid completes the electrical contact for the analyte liquidand permits the analyte to migrate out of the inner microtube and intothe sheath liquid. The sheath liquid, carrying the migrated analyte,forms a Taylor cone. In such a CE/MS embodiment, the mechanicalstability imparted by the invention provides stable liquid flow acrossthe tip and results in stable Taylor cone formation. Instability in flowand/or in the Taylor cone impairs performance and produces erraticsignal, signal drop out, noise, or any combination of performanceproblems. In FIG. 6, a protruding inner needle 13 portion is depicted.The needle protrusion is not essential to CE/MS but is useful inassisting Taylor cone formation.

FIG. 7 represents results obtainable by the invention taught herein inan APCI application. The line indicating droplet size 72 indicates theimprovement over prior art nebulizer performance 70.

The apparatus described above may be coupled to any analytical system inorder that droplets produced by the apparatus may be analyzed. Theapparatus of this invention provide a useful interface for analyticalsystems based on techniques such as mass spectrometry (MS), atomicabsorption (AA), or inductively coupled plasma (ICP) which cannotdirectly analyze liquid samples. In a preferred embodiment, theapparatus is coupled to a mass spectrometer. Any suitable massspectrometer may be used, for example quadrupole or multipole, magneticor electric sector, Fourier transform, ion trap, and time-of-flight massspectrometers. The invention is particularly useful as part of aninterface for mass spectrometers employing, for example, electrosprayionization (ESI) or atmospheric pressure chemical ionization (APCI) toanalyze liquid samples.

While this invention has been described with reference to severalembodiments, it is contemplated that various alterations andpermutations of the invention will become apparent to those skilled inthe art upon a reading of the preceding descriptions and a study of thedrawings. It is therefore intended that the scope of the presentinvention be determined by the following appended claims.

What is claimed is:
 1. An apparatus for forming droplets from a liquidcomprising:A. at least one inner microtube having an outer wall, an exitend, and an exit end aperture, B. an outer microtube having an innerwall, an exit end, and an exit end aperture, wherein the inner microtubehas an outer diameter smaller than the inner diameter of the outermicrotube, and the inner microtube is positioned within and issurrounded by the outer microtube such that an annular intermediatespace is formed therebetween, with the exit end of each microtube beinglocated at a same end of the apparatus, and wherein the exit end of theinner microtube is flush with respect to the exit end of the outermicrotube, C. one or more intermediate structures either (i) extendinginward radially from the inner wall of the outer microtube andcontacting the outer wall of the inner microtube for a predeterminedlength, (ii) extending outward radially from the outer wall of the innermicrotube and contacting the inner wall of the outer microtube for apredetermined length, or (iii) spanning the annular intermediate spaceand contacting both the outer wall of the inner microtube and the innerwall of the outer microtube for a predetermined length,wherein theintermediate structure is situated so as to mechanically stabilize theinner microtube, and D. one or more communicating channels continuinglengthwise along the outside of the inner microtube, wherein thecommunicating channel provides a continuation of the annularintermediate space and through which a fluid may continue to flow afterencountering the intermediate structure.
 2. An apparatus for formingdroplets from a liquid comprising:A. at least one inner microtube havingan outer wall, an exit end, and an exit end aperture, B. an outermicrotube having an inner wall, an exit end, and an exit end aperture,wherein the inner microtube has an outer diameter smaller than the innerdiameter of the outer microtube, and the inner microtube is positionedwithin and is surrounded by the outer microtube such that an annularintermediate space is formed therebetween, with the exit end of eachmicrotube being located at a same end of the apparatus, and wherein theexit end of the inner microtube protrudes beyond the exit end of theouter microtube, C. one or more intermediate structures either (i)extending inward radially from the inner wall of the outer microtube andcontacting the outer wall of the inner microtube for a predeterminedlength, (ii) extending outward radially from the outer wall of the innermicrotube and contacting the inner wall of the outer microtube for apredetermined length, or (ii) spanning the annular intermediate spaceand contacting both the outer wall of the inner microtube and the innerwall of the outer microtube for a predetermined length,wherein theintermediate structure is situated so as to mechanically stabilize theinner microtube, and D. one or more communicating channels continuinglengthwise along the outside of the inner microtube, wherein thecommunicating channel provides a continuation of the annularintermediate space and through which a fluid may continue to flow afterencountering the intermediate structure.
 3. A nebulizer assembly,comprising:A. a first inner microtube having an exit end and an exit endaperture, B. a second inner microtube having an exit end and an exit endaperture, wherein the first inner microtube has an outer diametersmaller than the inner diameter of the second inner microtube, and thefirst inner microtube is positioned within and is surrounded by thesecond inner microtube such that a first annular intermediate space isformed therebetween, C. an outer microtube having an exit end adaptedfor coupling with a tip and an exit end aperture, wherein the secondinner microtube has an outer diameter smaller than the inner diameter ofthe outer microtube, and the second inner microtube is positioned withinand is surrounded by the outer microtube such that a second annularintermediate space is formed therebetween, with the exit end of eachmicrotube being located at a same end of the nebulizer assembly, and D.a tip which couples with a surface at the exit end of the outermicrotube, thereby forming a region near the exit ends of the microtubeswithin which fluid may stabilize;wherein the inner diameter of the outermicrotube is narrowed for a predetermined portion of its length suchthat the second annular intermediate space is reduced to one or morefluid communicating channels.
 4. The nebulizer of claim 3, wherein atleast one of the first inner microtube, the second inner microtube, theouter microtube, and the tip has an outer surface which is chamfered,angled, or tapered.
 5. The nebulizer of claim 3, wherein the first innermicrotube, the second inner microtube, and the outer microtube each hasan outer surface which is chamfered, angled, or tapered.
 6. A massspectrometer system comprising the nebulizer of claim 3.